Reticulated ceramics, in other words, sintered ceramic foams have found numerous applications. They comprise a unique two-phase system. A continuous solid phase is interspersed in a continuous pore phase extending in all directions. The solid phase is made from a relatively inert ceramic material (high temperature resistant, inorganic materials, usually oxides, carbides, etc.). They are useful for filtering hot fluids such as diesel exhaust or liquid metals and as furnace insulation. Methods of producing a reticulated ceramic are disclosed in Schwartzwalder et al. U.S. Pat. No. 3,090,094 and British Pat. No. 916,784.
It is an object of this invention to provide reticulated ceramics having an integral, thin ceramic coating to close off the pore phase at selected locations and yet to retain the desirable properties of the reticulated ceramic. It is a special object of this invention to provide a ceramic article having a reticulated portion and a thin ceramic coating sintered together and matched to minimize thermal shock.
The thickness of the continuous layer formed in this way is unpredictable and tends to be many times the thickness of the webs comprising the reticulated portion. The coarser the pores of the reticulated portion (say less than thirty pores per inch) the more difficult it is to provide a thin continuous layer by the dipping process. Nevertheless, as may be expected, the surface layers formed by the dipping process are well knit to the reticulated layer being in contact with internal web surfaces. Applicants have found that an article with improved properties can be formed by a technique that results in much less contact between the web surfaces at the sealing layer. Moreover, the sealing layer will have an easily controlled uniform thickness. The articles made according to this invention will have improved thermal shock resistance due to the matching of the average thickness of the webs with the thickness of the ceramic sheets and the more than adequate adhesion between the surface and the reticulated portion.
It is often desirous for refractory materials to be inert to elevated temperatures, corrosive environments and rapid changes in temperature while maintaining its strength and structural integrity. It is further desirous to maximize these properties while minimizing heat capacity and thermal conductivity. There are many types of refractories available today ranging from the very dense fused cast types to the highly insulating fiber types. The fiber refractories have very low thermal conductivity and heat capacity which is desirous. The shortcomings of fiber refractories are low load bearing capability and low corrosion resistance along with shrinkage at the upper use temperature limits. The dense and insulating type refractories generally have good strength at temperature and are capable of being formed from corrosion and erosion resistant materials. The shortcomings of these materials, be it the preformed or monolith type, is that they have relatively high heat capacity due to their inherent mass. Due to the high heat capacity, the energy requirements to bring these materials to temperature is much greater than the fiber insulation.
It is an object of this invention to provide an improved refractory material, be it alone or in conjunction with fiber insulation, which will not compromise the desired load bearing properties and corrosion-erosion resistance for low heat capacity. This ceramic refractory structure or reticulated ceramic possesses good load bearing strength, corrosion-erosion resistance yet has a low thermal conductivity, low heat capacity and excellent thermal shock resistance.